It is known of course to provide vibration damping in hand-held tools and thereby to improve the use of the tool from the standpoint of increased operator comfort by minimizing the adverse effects of fatigue, carpal tunnel syndrome and the like. Prior examples of patented tools of the general type with which the present invention is concerned are the U.S. Patents to Honsa U.S. Pat. Nos. 4,648,468 and 4,771,833 and Honsa et al., U.S. Pat. No. 4,905,772. In the '772 design, an air motor is contained within a housing and isolated from relative vibration by an elastomer sleeve of molded form telescoped over the motor and disposed between the exterior of the motor and the interior surface of the housing. The sleeve is a separate element slipped onto the motor during assembly. The sleeve fits the motor rather tightly and is interlocked with the housing by elastomer key means. Further, handles attached to the housing are outwardly divergent from the housing and lie in a plane normal to the axis of rotation of the motor shaft, requiring that the user grip the handles with his wrists bent downwardly, a position found to be conducive to the creation of carpal tunnel syndrome, the characteristics of which are that the tendons passing through the narrow channel in the wrist are overused and press on the median nerve that controls feeling in the hand. The disorder is extremely painful and can be permanent.
According to the present invention, the foregoing and other disadvantages of prior structures are eliminated by an improved design in which a motor housing is contained within a dome-like casing and isolated from the casing by poured-in elastomer vibration and torque isolating material that fills the bell-shaped space between the interior surface of the casing and the outer surface of the motor housing. Further, the elastomer is introduced in laminar form, or layer by layer, each layer being allowed to at least partially set up before the introduction of subsequent layers. A further feature is that not all layers are of the same Shore A hardness, which further adds to the efficacy of the isolation. It is significant that the handles attached to the outer casing extend upwardly and forwardly with respect to a user standing behind the tool, enabling the user to grasp the handles with his wrists fairly straight, with his fingers wrapped about the grips and with his thumbs uppermost, positions determined to be instrumental in eliminating or materially reducing fatigue and the effects of carpal tunnel syndrome.
Features in addition to the foregoing will become apparent as a preferred embodiment of the invention is disclosed in the ensuing description and accompanying drawings.